This is kind of a weird mix of UNIX/Emacs/bash and traditional Atari.

ASCII characters:

Shift-Escape - backtick: `
Ctrl-Escape - tilde: ~
Ctrl-comma - left curly brace: {
Ctrl-period - right curly brace: }

Cursor movement:

Left/Right arrows - move cursor left/right
Up arrow - if the inputbox has anything in it, move up 1 line,
   aka 40 characters, or to the start if the text is less than
   40 characters.
Up arrow *in an empty inputbox* - bring up last entered command.
   If this doesn't seem to work, make sure your input box really is
   empty (spaces are invisible, but count as "not empty"). If in
   doubt, press Shift-Del.
Down arrow - moves down 1 line, aka 40 characters. If this would
   move past the end of the text, moves to the end.
Ctrl-A - Move to start of buffer.
Ctrl-E - Move to end of buffer.
Ctrl-F or Ctrl-Shift-Up - move right by one word.
Ctrl-B or Ctrl-Shift-Down - move left by one word.

Deleting text:

Backspace - Delete the character to the left of the cursor. In insert
   mode, it closes up the gap. In typeover mode, it leaves a space
   where the deleted character was (like BASIC does).
Shift-Del - Delete (clear) buffer.
Shift-Clear or Ctrl-Clear: clear buffer and hide input box (show status).
Ctrl-Del or Ctrl-X - delete the character under the cursor.
Ctrl-U - Delete to start of buffer.
Ctrl-W - Delete word to left of cursor
Ctrl-U - Delete to start of buffer
Ctrl-K - Kill (delete) to end of buffer.

Formatting:

Atari/Inverse key - Toggle bold (displays as inverse B).
Ctrl-I - Toggle italic (displays as down-arrow).
Ctrl-shift-U - Toggle underline (displays as right-arrow).

Other:

Tab - Nick/channel completion (only at the start of an empty input box!)
Ctrl-Insert - Toggle insert/typeover.
Escape - Treats next keystroke as though Start were held down.
   Pressing Escape again gets out of this mode.


Future plans:

^Y, Shift-Insert - paste (^K, ^U, ^W fill a paste buffer; need RAM)
Shift-Return: Maybe... send buffer but do not clear it.

^S - toggle strikethrough
